Achieving two-photon fluorescence bioimaging and photodynamic therapy for D–A conjugated polymers through manipulating twisted intramolecular charge transfer

Abstract

Two-photon fluorescence bioimaging holds immense promise for deep tissue imaging. However, its widespread application is hampered by the relatively low fluorescence quantum yields (ϕ_PL) and small two-photon absorption cross-sections (δ₂). Herein, we report the achievement of high ϕ_PL for donor–acceptor alternating conjugated polymers comprising dibenzothiophene-S,S-dioxide derivatives as electron acceptors and a triphenylamine derivative as an electron donor, which is accomplished by manipulating twisted intramolecular charge transfer (TICT). The resulting polymer PDA5 exhibits intensified green-light emission with a high ϕ_PL value of 70.7% in toluene. Its corresponding nanoparticles (PDA5 NPs), which are fabricated by the nanoprecipitation method, display a uniform size of around 83 nm, a large two-photon absorption cross-section (δ₂) of 5.03 × 10⁵ GM in water upon excitation by an NIR laser (840 nm), and a moderate ϕ_PL value of 40.7% in aqueous medium. These nanoparticles also present good photostability and outstanding two-photon fluorescence imaging with penetrating bladder blood vessels of 225 μm as well as real-time monitoring. Furthermore, PDA5 nanoparticles with impressive reactive oxygen species generation capability under irradiation of a white light source are implemented for photodynamic therapy in in vitro cancer cells. The observations manifest that efficient conjugated polymers with high ϕ_PL and large δ₂ can be materialized for two-photon fluorescence imaging and photodynamic therapy by manipulating twisted intramolecular charge transfer.